1. Field of the Invention
The present invention relates to a joined structure of different metal materials that are combined in such a manner as to form a fragile reaction product layer at a joined interface, such as a directly joined structure between a steel product and an aluminum alloy.
2. Description of the Related Art
In general, different metal materials tend to metallurgically form very fragile intermetallic compounds. In the case of the use of a normal melt joining process, since the two materials are mixed in a liquid state, a large amount of fragile reaction product (intermetallic compound) is formed on a melt-joined metal portion, failing to provide a joint having an appropriate strength. In contrast, in the case of application of a solid-phase joining method, it is possible to reduce the generation of a reaction product in comparison with the melt joining method; therefore, various joining methods of this type have been proposed.
However, the joining mechanism, that is, the relationship between the joined interface structure and the joining strength, has not been clarified, and at present, this method does not provide joints having high strength.
The present invention has been devised to solve the above-mentioned problems, and an object thereof is to provide a joined structure having a high strength upon manufacturing a joint made of different metal materials by taking into consideration the generation state of reaction products at the joining interface.
One aspect of the joined structure of different metal materials of the present invention is characterized in that, in a joined structure different metal materials, the thickness of a reaction product layer to be generated at the joined interface is set to be 0.5 xcexcm or less.
Another aspect of the joined structure of different metal materials of the present invention is characterized in that a reaction product layer is located at the joined interface intermittently.
Moreover, another aspect of the joined structure of different metal materials of the present invention is characterized in that a base material crystal consisting of base material atoms of 90 atom % or more is contained in the reaction product layer generated at the joined interface. Moreover, another aspect of the joined structure of different metal materials of the present invention is characterized in that the base material crystal penetrates the reaction product layer.
Furthermore, another aspect of the joined structure of different metal materials of the present invention is characterized in that an oxide layer is placed at the joined interface by taking the effectiveness of the oxide layer at the joined interface into consideration. In another aspect of the present invention, the thickness of the oxide layer is limited to a range of 1 to 30 nm so as to more preferably obtain a joined structure having high strength.
The reaction product of the present invention is generated by a diffusion reaction of mutually joined material atoms at the joined interface, and in the case of a combination of different metal materials, in general, an intermetallic compound which is more fragile than the base material is formed. Moreover, in most cases, the structure is a polycrystal structure, and the generation state, which differs depending on joining methods and joining conditions, is rate-controlled by a diffusion reaction; therefore, the amounts generated become greater as the reaction temperature and time increase. When this reaction product layer is formed excessively, a large area having low toughness tends to spread on the joined interface, resulting in an increase in the probability of rupture in response to loads; consequently, the joint is easily damaged.
In the present invention, the oxide layer refers to a residual oxide layer at the surface of the joining members or an oxide layer generated during the joining process, and the two members are joined to each other through the oxide layer on the joining interface. This oxide layer must have a thin structure to a degree so as not to exert adverse effects on the welding properties and the joining strength between the two joining members. Without the presence of this oxide layer, when the surfaces of the members that have been sufficiently cleaned are made to contact with each other during a solid-phase joining process, the base material atoms of the two joining members will mutually diffuse, easily forming a very fragile reaction product layer.
The rupturing conditions of the joints are determined by the balance between the probability of cracks occurring and developing in the reaction product layer having low toughness and the probability of these occurring and developing in the base material having high ductility. In other words, in order to increase the strength of the joining portion, it is possible to lower the probability of rupturing inside the reaction product layer by reducing the amount of generation of the reaction product layer having low toughness. When the probability of rupturing inside the reaction product layer is set to be lower than that in the joining base material, the rupturing does not occur in the joining interface, but in the joining base material. In other words, such a joint is less susceptible to rupturing inside the reaction product layer, and it has high strength.
Therefore, the joined structure of different metal materials in the present invention is formed as a joined structure, shown in the respective embodiments in FIGS. 1 to 4, so that it becomes possible to achieve the formation of joints having high strength. The following description will explain the respective embodiments of the joined structures of different metal materials of the present invention in detail.
(1) First Embodiment
As shown in FIG. 1, a joined structure of different metal materials of the first embodiment of the present invention, which is a joined structure between a first member 1 and a second member 2 that generates a fragile reaction product layer at the joined interface, is characterized in that the generation of the reaction product having low toughness at the joining interface is reduced to a minimum, with the thickness of the reaction product layer 3 being set to 0.5 xcexcm or more; thus, it becomes possible to reduce the probability of the generation of cracks (probability of rupturing) inside the layer to be set to not more than the probability of rupturing inside the second member serving as a joining base material, and consequently, to provide a joint with high strength that is less susceptible to rupturing at the joined interface.
(2) Second Embodiment
As shown in FIG. 2, a joined structure of different metal materials of the second embodiment of the present invention, which is a joined structure between a first member 1 and a second member 2 that generates a fragile reaction product layer on the joined interface, is characterized in that the generation of the reaction product having low toughness at the joining interface is reduced to a minimum, with the reaction product layer 3 being located intermittently; thus, cracks generated inside the reaction product having low toughness are stopped by the base material having high ductility in the intermittent portions; therefore, it becomes possible to reduce the probability of rupturing thereof to be not more than the probability of rupturing inside the joined base material; and consequently, it becomes possible to provide a joint with high strength that is less susceptible to rupturing from the joined interface. Moreover, with respect to the intermittence of the reaction product layer of the present invention, a proper distance capable of maintaining a sufficient ductility portion is provided so as to stop cracks occurring inside the reaction product layer.
(3) Third Embodiment
As shown in FIG. 3, a joined structure of different metal materials of the third embodiment of the present invention, which is a joined structure between a first member 1 and a second member 2 that generates a fragile reaction product layer at the joined interface, is characterized in that the generation of the reaction product having low toughness at the joining interface is reduced to a minimum, with a base material crystal 4 containing 90 atom % or more of the base material atoms being located in the reaction product layer 3. More preferably, the base material crystal 4 exists so as to penetrate the reaction product layer 3. Since this base material crystal contains 90 atom % or more of the base material atoms, it is allowed to have virtually the same ductility as the joining base material. With this arrangement, cracks occurring in the reaction product having low toughness are stopped by this base material crystal so that the probability of rupturing in this joining interface is reduced to not more than the probability of rupturing inside the joining base material; thus, it becomes possible to obtain a joint having high strength that is less susceptible to rupturing from the joined interface.
(4) Fourth Embodiment
As shown in FIG. 4, a joined structure of different metal materials of the fourth embodiment of the present invention, which is a joined structure between a first member 1 and a second member 2 that generates a fragile reaction product layer on the joined interface, is characterized in that, when the mutual diffusing coefficient of the main element of an oxide layer 5 located on the interface is less than the mutual diffusing coefficients in both of the base materials, this oxide layer 5 has an effect for suppressing the generation of the reaction product at the joined interface; therefore, by placing a very thin oxide layer 5 at the joined interface, atom diffusions from both of the joining members are suppressed, with the result that the generation and development of a fragile reaction product which would be formed at the interface are suppressed, so that it becomes possible to obtain a joined structure with high strength made of different metal materials. In order to obtain a sufficient function of this oxide layer serving as an atom diffusion barrier, the thickness of the layer must be set to at least 1 nm.
Here, in the case in which an oxide layer is formed at the joined interface, the thickness of the oxide layer exceeding 30 nm sometimes causes degradation in the welding properties of the joining members, and generates and develops cracks inside the oxide layer that is more fragile than the base material. Therefore, it is necessary to make the oxide layer of the joining members thin to a degree so as not to cause adverse effects on the welding properties and the toughness of the joined interface; thus, it is possible to function as an atom diffusion barrier to both of the joining members, and consequently to reduce the formation and development of the interface reaction layer. Therefore, in the present invention, the thickness of the oxide layer is preferably set in a range of 1 to 30 nm. In this case, a reaction product which is sufficiently small to a degree so as not to yield adverse effects on the joining strength may exist in contact with the oxide layer.
(5) Formation of a Joined Interface
The joined structures of different metal materials of the respective embodiments having the above-mentioned joined interfaces are formed by, for example, a friction welding method, which is one type of solid-phase joining method. In the frictional process, the surface of the joining member is mechanically cleaned, and in the succeeding upset process, reaction products generated at the joined interface are externally discharged, thereby completing welding of both of the joining members. If the frictional process is carried out insufficiently, the joining faces are not sufficiently cleaned, causing a state in which stains and residual oxides are excessively left at the joining faces and results in failure in providing a desirable adherence in the succeeding upset process. In contrast, if the frictional process is excessively carried out, although the joining faces are sufficiently cleaned, too much input heat is given to the joining members, causing the reaction product layer to grow extremely in the upset process.
Moreover, in the fourth embodiment, with respect to the means for providing a thin oxide layer on the joined interface, a method in which only one of the oxide layers is removed from the oxide layers existing on the surfaces of both of the base materials before the joining operation is proposed. In accordance with this method, it is possible to obtain a joined interface having a thinner oxide layer in comparison with the case in which oxide layers exist on the surfaces of both of the base materials. Moreover, in the above-mentioned method, only one of the oxide layers that is thicker may be preferably removed. With this arrangement, it is possible to obtain a joined interface having a thinner oxide layer in comparison with the cases in which oxide layers exist on the surfaces of both of the base materials and only the thicker oxide layer remains.
With respect to the means for removing only one of the oxide layers of the base materials, before the joining process or during the joining process, a mechanical means such as grinding, rubbing, and sliding, a physical means such as sputtering, or a chemical means such as reduction, may be used, and after this process, making the cleaned face without the oxide layer and a joining face bearing a thin oxide layer in contact with each other to such a degree as to exert a bonding strength between the atoms on both of the joined faces; thus, it is possible to form the joined structure of different metal materials of the present invention. Moreover, the oxide layer of the present invention may be reduced by active elements contained in the joined members during the joining process so that an oxide layer newly containing the active elements is formed.
(6) Confirmation of the Joined Structure
In the joined structure of different metal materials of the present invention, the fact that the members of different metals are in a joined state can be observed and confirmed through a microscopic observation to detect the existence of a reaction product layer made of an intermetallic compound at the joined interface; therefore, it is possible to eliminate the need for complex strength tests and inspections.